Satellite radiotelephone communications systems are used widely to provide dependable, high quality communications. See, for example, U.S. Pat. No. 5,303,286 to Robert A. Wiedeman, entitled Wireless Telephone/Satellite Roaming System.
FIG. 1 is a conceptual diagram of a satellite radiotelephone system. As shown in FIG. 1, a satellite radiotelephone system typically includes one or more satellites 110, which may serve as relays or transponders between at least one central earth station 130 and radiotelephones 120. The earth station may in turn be connected to a public switched telephone network 140, allowing communications between satellite radiotelephones, and communications between satellite radiotelephones and conventional terrestrial cellular radiotelephones or landline telephones. The satellite radiotelephone system may utilize a single antenna beam covering the entire area served by the system, or, as shown in FIG. 1, the satellite may be designed such that it produces multiple beams 150, each serving distinct geographical coverage areas 160 in the system's service region. Thus, a cellular architecture similar to that used in conventional terrestrial cellular radiotelephone systems can be implemented in a satellite-based system. The satellite 110 typically communicates with a radiotelephone 120 over a bidirectional communications pathway, with radiotelephone communications signals being communicated from the satellite 110 to the radiotelephone 120 over a downlink (or forward link) 170, and from the radiotelephone 120 to the satellite 110 over an uplink (or reverse link) 180.
Satellite radiotelephone systems are increasingly being developed for areas where the small number of thinly scattered users and/or the rugged topography may make conventional landline telephone or cellular telephone infrastructure technically or economically impractical. Unfortunately, many of the natural features which may make it commercially impractical to install conventional landline or cellular telephone infrastructures may also impede signals traveling between radiotelephones and satellites. Dense foliage, hills, mountain ranges, and adverse weather conditions may all impede the relatively weak signals transmitted by satellites and radiotelephones.
Satellite radiotelephone systems may also serve urban areas where large buildings and other man-made structures may contribute to the degradation of satellite telecommunications signals caused by any naturally occurring features. It is also expected that a radiotelephone user will desire to use the radiotelephone while in a building or while riding in an automobile or other vehicle. The structure, mechanical operation and electrical circuitry present in an automobile or any other vehicle, and the structure of a building, along with any electrical or mechanical systems found therein or nearby, may further weaken signals traveling between satellites and radiotelephones.
The relatively small link margins involved in the typical satellite telecommunications system may be further decreased by a host of other physical processes. Both the forward and return link signals may be subject to numerous degrading conditions. For example, communications links between transmitters and receivers in mobile communications systems may be affected by channel fading. Link degradation is generally caused by a combination of effects including multipath fading, Doppler shift due to the movement of the receiver relative to the transmitter, and additive noise.
Multipath fading is the result of a multipath channel existing whenever there is more than one path for the transmitted energy to travel between transmitter and receiver. For example, a satellite communications downlink may have a direct path between the satellite and the mobile user plus a reflected path from the ground or from other structures. In general, the reflected path will arrive at the receiver out-of-phase with the direct path and the amplitude of the received signal will be attenuated by the multipath effect. As the mobile user travels, the phase difference will generally change, resulting in a time-varying amplitude at the receiver. Multipath channel fading as predicted by the Rician channel model (when the line-of-sight path is strong compared to the reflected paths), is expected to consume from 3 to 5 decibels of the available link margin. Additionally, just the proximity of a human body to the radiotelephone may also degrade the link margin by from 2 to 4 decibels.
In order to provide robust, stable communications, a satellite telecommunications system should generally provide signals of adequate strength to penetrate both natural and man-made obstacles which are in the signal path between the satellite and the radiotelephone. Unfortunately, satellites such as those comprising the Mobile Satellite System (MSS) now being implemented in the United States by the American Mobile Satellite Corporation, are generally severely power limited so it may be difficult to offer forward link margins that compare favorably to those offered by terrestrial cellular and/or Personal Communication Service (PCS) base stations. Likewise, the radiated power levels of radiotelephones are also generally limited due to the size limitations of hand-held radiotelephones and power consumption limitations (as a function of battery charge life). Unfortunately, typical satellite antennas planned for deployment by MSS satellites are also generally size limited and thus may not be able to compensate for the low transmission power of the uplink signal transmitted by the radiotelephone. The result is both forward and reverse link margins which may be relatively small.
The MSS satellites presently planned for deployment in the 1998-2000 time frame are only expected to provide link margins on the order of 8-16 dB above the ideal additive white gaussian noise (AWGN) channel. Such link margins may necessitate an almost completely unobstructed line-of-site (LOS) path between the radiotelephone and the satellite with little link margin left over to compensate for shadowing and or blockage caused by terrain, trees, foliage, and buildings.